There is a current trend to integrate photonic devices and electronic devices on the same semiconductor substrate. A silicon-on-insulator (SOI) substrate can be used as the supporting substrate for such integration. However, photonic structures such as waveguides built over an SOI substrate generally require a thick buried oxide (BOX) in the SOI substrate for optical isolation compared with a thinner BOX typically required for electrical isolation of electronic devices. For example, to prevent evanescent coupling of a photonic waveguide core to the supporting silicon beneath the BOX, the BOX material must be relatively thick, for example, greater than 1.0 μm and often 2.0 μm-3.0 μm thick. When the BOX material has such a thickness, it inhibits heat flow to the underlying silicon, the latter of which acts as a heat dissipator for both electronic and photonic devices. By comparison, when certain electronic devices, such as high speed logic circuits, are integrated on the same SOI substrate as photonic devices, the BOX of the SOI substrate should be much thinner, for example, in the range of 100-200 nm. Such BOX insulator, while providing a good SOI substrate for electronic devices, is insufficient to prevent optical coupling of the waveguide core to the underlying supporting silicon of the SOI substrate, which causes undesirable optical signal loss. Accordingly, a complex multiple-mask process is required to provide an SOI substrate, or non-SOI substrate, which has suitable electrical and optical isolation having different depths in different areas of the substrate for electronic and photonic devices.
In addition, although SOI substrates are often used for fabrication of electronic devices and photonic devices on the same substrate, SOI substrates are relatively expensive to produce compared with non-SOI substrates and can often also be of limited supply.
Accordingly, a simplified method is needed for providing a common semiconductor substrate having isolation areas of different depths for isolating electronic and photonic devices.